In modern radio networks, an important tool for the efficient use of the radio spectrum is the careful control of the radiation patterns of base station antennas in both the azimuth and elevation planes. The radiation pattern of an antenna array is characterized by a main beam and subsidiary beams known as sidelobes. The main beam is arranged to illuminate the desired coverage area. The main beam has a defined direction relative to the physical axis of the antenna array and a beamwidth, usually defined as the angle in the azimuth or elevation plane between points having a radiation intensity of one half the maximum intensity. The subsidiary beams or sidelobes may cause interference to the service provided by other base stations and must therefore be reduced in magnitude to mitigate such interference.
An active phased antenna array comprises a plurality of radiating elements wherein each radiating element is connected to radio transmitters and/or receivers. The connection to each radiating element may include phase shifting circuitry to allow the direction and shape of the radiation pattern of the array to be varied by means of analog or digital control signals. This technology has been employed for military uses in the past but more recently is being employed for mobile radio base stations, providing a means by which the coverage and capacity of a network may be increased. However, the acceptance of this technology has been restricted by the high cost of radios with beam steering functions. This is at least partly due to the additional cost of providing phase shifting circuitry or other beam-steering circuitry for each individual radiating element.
FIG. 1 shows a prior art N-element phased array in schematic form. In this arrangement the signal contributions from all elements will arrive in phase at a distant point in the direction of the main beam maximum. The direction of the main beam may be varied by the choice of the differential phase shift between adjacent antenna elements. In accordance with the principle of reciprocity, the same differential phase shifts at a given frequency will result in the same main beam direction for both the transmission and reception of radio signals. In the following description specific reference is made to vertical beam steering, but the method herein described may be applied to a vertical array of elements, providing beam steering in the elevation (tilt) plane, or to a horizontal array when steering will be in the azimuth plane. It may also be applied to a planar array in which case beam steering may be applied to both planes.
In addition to applying a linear phase shift to the currents in the elements of the array, the relative amplitudes and relative phases of the currents may be further optimised. For example, the amplitudes of the currents fed to array elements may be arranged in such a manner that the elements near the ends of the array have lower currents than those near the centre of the array. Various methods for achieving this objective are well known (for example, see Chapters 3, 20 and 29 of the Antenna Engineering Handbook, J L Volakis, editor, 4th Edition, McGraw Hill, New York, 2007).
FIG. 2 shows a typical circuit arrangement for the phased array of FIG. 1. Based on the application of equal differential phase shifts for a five-element array, FIG. 3 shows the radiation patterns at 0°, 10° and 20° from the array normal direction. As can be seen, for sidelobes within 30° of the main beam, the sidelobes are lower than the value required by mobile operators today in urban areas (typically at least 18 dB below the main beam level). However, this approach is hugely expensive. The electronic phase shifters, good quality mixers and also the transmit modules, which include the main components like power amplifiers (PAs), band pass filters (BPFs), pre-PAs, tuning circuits and heatsinks are very expensive and represent a large proportion of the cost of the array.
An existing method by which the number and cost of active components in an array may be reduced is to group at least some of the elements into subarrays, each typically comprising two elements. In such an arrangement, the differential phase between the members of each subarray is fixed, and is typically optimised for the mean value of the required tilt range. However, such techniques are typically beamtilt-limited because it is only possible to dynamically adjust the relative phases between the subarrays and not within them. As the tilt move towards the extremes of its range, the sidelobe performance degrades considerably because the differential phase shift between adjacent elements of the whole array is not linear.
By way of example, FIG. 4 shows a five element array divided into subarrays comprising 2, 1 and 2 elements respectively. The phase difference between the members of the outer pairs of elements can be optimised for the mid-tilt angle, which in this example is 10°, and accordingly the phase difference is fixed at 44°. However, as the beam is moved away from a tilt of 10° by applying a linear phase shift between the subarrays, the sidelobes become higher. Through the use of this arrangement, the number of costly components (e.g. transmit modules and mixers) has been reduced, but the sidelobe performance, as seen in FIG. 5, is unacceptable in a mobile network, especially in densely populated areas.